Solid-state Lithium batteries were developed by Duracell in the 1970's and made commercially available in the 1980's but are no longer produced. The cells used a lithium metal anode, a dispersed phase electrolyte of lithium iodide and Al2O3 and a metal salt as the cathode. The Li/LiI(Al2O3)/metal salt construction was a true solid-state battery. These batteries were not rechargeable.
It has long been a goal to develop a rechargeable solid state lithium-based battery using inorganic solid electrolyte material because of the passivation reactions and unstable interfaces that form between organic electrolyte materials such as liquid and solid polymer electrolytes. In the early 1990's another all-solid-state battery was developed at the Oak Ridge National Laboratories. These cells consist of thin films of cathode, inorganic electrolyte, and anode materials deposited on a ceramic substrate using vacuum deposition techniques including RF sputtering for the cathode and electrolyte, and vacuum evaporation of the Li metal anode. The total thickness of the cell is typically less than 10um with the cathode being less than 4um, the solid electrolyte around 2um (just sufficient to provide electrical isolation of the cathode and anode) and the Li anode also around 2um. Since strong chemical bonding (both within each layer and between the layers of the cell) is provided by the physical vapor deposition technique, the transport properties are excellent. Although the solid electrolyte LiPON has a conductivity of only 2×10−6 S/cm-1 (fifty times lower than that of the LiI(Al2O3) solid electrolyte used in the Duracell battery described above), the impedance of the thin 2um layer is very small allowing for very high rate capability. Batteries based on this technology are very expensive to fabricate, are very small, and have very low capacity.
Currently, Li-ion battery chemistry gives the highest performance and is becoming the most widely used of all battery chemistries. The cells consist of thick (˜100um) porous composite cathodes cast on a thin (˜10um) Al foil current collector. The composite cathode typically contains LiCoO2 as the active material due to its high capacity and good cycle life, and carbon black to provide electrical conductivity throughout the layer. A thin polymer separator is used to provide electrical isolation between the cathode and the carbon based anode which intercalates Li during the charge cycle. The cell is immersed in liquid electrolyte which provides very high conductivity for the transport of Li ions between the cathode and anode during charge and discharge. Because the thick composite cathode is porous, the liquid electrolyte is absorbed into and fills the structure, and thus provides excellent surface contact with the LiCoO2 active material to allow fast transport of Li ions throughout the cell with minimal impedance.
The liquid electrolyte itself consists of a Li salt (for example, LiPF6) in a solvent blend including ethylene carbonate and other linear carbonates such as dimethyl carbonate. Despite improvements in energy density and cycle life, there remains an underlying problem with batteries that contain liquid electrolytes. Liquid electrolytes are generally volatile and subject to pressure build up explosion and fire under a high charge rate, a high discharge rate, and/or internal short circuit conditions. Charging at a high rate can cause dendritic lithium growth on the surface of the anode. The resulting dendrites can extend through the separator and cause a short circuit in the cell. The self-discharge and efficiency of the cell is limited by side reactions and corrosion of the cathode by the liquid electrolyte. The liquid electrolyte also creates a hazard if the cell over-heats due to overvoltage or short circuit conditions creating another potential fire or explosion hazard.
To address safety and reliability problems with lithium based batteries that employ liquid electrolytes, and to achieve high energy density, solid-state batteries that employ high capacity lithium intercalation compounds are being developed. These all-solid-state batteries consist of a composite cathode containing active battery cathode material (e.g., LiNiMnCoO2, LiCoO2, LiMn2O4 Li4Ti5O12 or similar), an electrically conductive material (e.g., carbon black), and lithium ion conductive glass electrolyte material, such as Li3xLa2/3-xTiO3 (x=0.11) (LLTO) or Li7La3Zr2O12 (LLZO) that is formed in situ from a liquid precursor via a low temperature sol gel process. When gelled and subsequently cured, the precursor is transformed into a solid lithium ion conductive glass electrolyte. Past attempts at constructing such all-solid-state batteries have been limited by the need to bind the materials together in order to facilitate effective lithium ion transport across interfaces. This binding process has been attempted by sintering at high temperature. The temperatures required for effective sintering are in the range of 600° C. and higher. The problem has been that the cathode and electrolyte materials will react with each other at such sintering temperatures resulting in high impedance interfaces and an ineffective battery.
In constructing a solid-state battery using the low temperature sol gel approach, a cathode is formed by mixing a lithium active material, an electrically conductive material, and the liquid sol gel precursor to form a slurry or paste. The cathode can be formed as either a thick pellet or as a thin casting containing the mixture of cathode components. The cathode is held together by the ion conductive glass electrolyte matrix that is formed by gelling and curing the sol-gel precursor solution. Curing temperature for the gelled precursor is in the range of 300° C., thus parasitic reactions are avoided.
Construction of battery electrodes using the sol gel approach to produce glass electrolyte as a binder requires proper gelling, drying, and curing of the precursor. Gelling of precursors for LLTO and LLZO is a hygroscopic process. Moisture must diffuse into the cathode structure through the tortuous path formed by the densely packed cathode powder materials in order for the cathode material to gel properly throughout. Secondly, drying of the precursor after gelling can be time consuming because solvents and alcohols must diffuse through the gelled electrolyte within the tortuous compacted electrode powder structure.
The all-solid-state primary cell developed by Duracell and described in detail above demonstrated very high energy densities of up to 1000 Wh/L and excellent performance in terms of safety, stability, and low self-discharge. However, due to the pressed powder construction and the requirement for a thick electrolyte separation layer, the cell impedance was very high, severely limiting the discharge rate of the battery. This type of cell is also restricted in application because the electrochemical window is limited to less than three volts due to the iodide ions in the electrolyte which are oxidized above approximately three volts. In addition, a stable rechargeable version of this cell was never developed.
The all-solid-state thin film battery developed by Oak Ridge National Laboratories, also detailed above, addresses many of the problems associated with Li-ion technology, but also has limitations of its own. The vacuum deposition equipment required to fabricate the cells is very expensive and the deposition rates are slow leading to very high manufacturing costs. Also, in order to take advantage of the high energy density and power density afforded by use of the thin films, it is necessary to deposit the films on a substrate that is much smaller and lighter than the battery layers themselves so that the battery layers make up a significant portion of the volume and weight of the battery compared to the inert substrate and packaging components. It is not practical to simply deposit thicker layers as the cathode thickness is limited because lithium diffusion rates within the active material limit the thickness of a cathode that can be charged and discharged at useful rates. Therefore the films must be deposited on very thin substrates (<10um) or multiple batteries must be built up on a single substrate, which leads to problems with maintaining low interface impedance with the electrolyte during the required high temperature annealing of the cathode material after deposition.